Methods and apparatus to generate liquid ambulatory oxygen from an oxygen concentrator

ABSTRACT

The present invention is directed to a much safer and less expensive way of providing portable oxygen from a gas concentrator for patients who do not want to be tied to a stationary machine or restricted by present oxygen technology. In one preferred embodiment, the present invention splits off some of the excess capacity gas flow from a gas concentrator which is then stored via liquefaction. The stored gas can then be used as a portable supply. A portion of the oxygen gas flow generated by the oxygen concentrator is channeled to a condenser which receives and liquefies the oxygen gas using cryocooler. A storage dewar is used for storing the oxygen liquefied by the condenser. Liquid is then selectively transferred to a smaller portable dewar. A controller can be used for monitoring the parameters of liquefaction, including oxygen concentration, the amount of liquid oxygen in the dewar, and for controlling the parameters of liquid oxygen generation and transfer. In one embodiment, the flow rate into the condenser is chosen to exceed the capacity of the condenser to minimize the liquefaction of argon, nitrogen and trace gases, and to purge the system.

BACKGROUND OF THE INVENTION

The field of this invention relates to using an oxygen concentrator tocreate a portable supply of supplementary oxygen for ambulatoryrespiratory patients so that they can lead normal and productivelives--as the typical primary oxygen sources are too bulky to carry orrequire excessive power to operate.

There is a burgeoning need for home and ambulatory oxygen. Supplementaloxygen is necessary for patients suffering from lung disorders; forexample, pulmonary fibrosis, sarcoidosis, or occupational lung disease.For such patients, oxygen therapy is an increasingly beneficial,life-giving development. While not a cure for lung disease, supplementaloxygen increases blood oxygenation, which reverses hypoxemia. Thistherapy prevents long-term effects of oxygen deficiency on organsystems--in particular, the heart, brain and kidneys. Oxygen treatmentis also prescribed for Chronic Obstructive Pulmonary Disease (COPD),which afflicts about 25 million people in the U.S., and for otherailments that weaken the respiratory system, such as heart disease andAIDS. Supplemental oxygen therapy is also prescribed for asthma andemphysema.

The normal prescription for COPD patients requires supplemental oxygenflow via nasal cannula or mask twenty four hours per day. The averagepatient prescription is two liters per minute of high concentrationoxygen to increase the oxygen level of the total air inspired by thepatient from the normal 21% to about 40%. While the average oxygen flowrequirement is two liters per minute, the average oxygen concentratorhas a capacity of four to six liters of oxygen per minute. This extracapacity is occasionally necessary for certain patients who havedeveloped more severe problems but they are not generally able to leavethe home (as ambulatory patients) and do not require a portable oxygensupply.

There are currently three modalities for supplemental medical oxygen:high pressure gas cylinders, cryogenic liquid in vacuum insulatedcontainers or thermos bottles commonly called "dewars," and oxygenconcentrators. Some patients require in-home oxygen only while othersrequire in-home as well as ambulatory oxygen depending on theirprescription. All three modalities are used for in-home use, althoughoxygen concentrators are preferred because they do not require dewarrefilling or exchange of empty cylinders with full ones.

Only small high pressure gas bottles and small liquid dewars areportable enough to be used for ambulatory needs (outside the home).Either modality may be used for both in-home and ambulatory use or maybe combined with an oxygen concentrator which would provide in-home use.

As we describe below, the above-described current methods and apparatushave proven cumbersome and unwieldy and there has been a long-felt needfor improved means to supply the demand for portable/ambulatory oxygen.

For people who need to have oxygen but who need to operate away from anoxygen-generating or oxygen-storage source such as a stationary oxygensystem (or even a portable system which cannot be easily carried), thetwo most prescribed options generally available to patients are: (a) tocarry with them small cylinders typically in a wheeled stroller; and (b)to carry portable containers typically on a shoulder sling. Both thesegaseous oxygen and liquid oxygen options have substantial drawbacks. Butfrom a medical view, both have the ability to increase the productivelife of a patient.

The major drawback of the gaseous oxygen option is that the smallcylinders of gaseous oxygen can only provide gas for a short duration.Oxygen conserving devices that limit the flow of oxygen to the time ofinhalation may be used. However, the conserving devices add to the costof the service and providers have been reluctant to add it because thereoften is no health insurance reimbursement. Indeed, the insurancereimbursement for medical oxygen treatment appears to be shrinking.

Another drawback of the gaseous oxygen option is the source of or refillrequirement for oxygen once the oxygen has been depleted from thecylinder. These small gas cylinders must be picked up and refilled bythe home care provider at a specialized facility. This requires regularvisits to a patient's home by a provider and a substantial investment insmall cylinders for the provider because so many are left at thepatient's home and refilling facility. Although it is technicallypossible to refill these cylinders in the patient's home using acommercial oxygen concentrator that extracts oxygen from the air, thistask would typically require an on-site oxygen compressor to boost theoutput pressure of the concentrator to a high level in order to fill thecylinders. Additionally, attempting to compress the oxygen inpressurized canisters in the home is dangerous, especially for untrainedpeople. This approach of course presents several safety concerns forin-home use. For example, in order to put enough of this gas in aportable container, it must typically be compressed to high pressure(˜2000 psi). Compressing oxygen from 5 psi (the typical output of anoxygen concentrator) to 2000 psi will produce a large amount of heat.(Enough to raise the temperature 165° C. per stage based on threeadiabatic compression stages with intercooling.) This heat, combinedwith the oxygen which becomes more reactive at higher pressures, sets upa potential combustion hazard in the compressor in the patient's home.Thus, utilizing and storing a high pressure gas system in the patient'shome is dangerous and not a practical solution.

The convenience and safety issues are not the only drawbacks of thiscompressed oxygen approach. Another drawback is that the compressors orpressure boosters needed are costly because they require special careand materials needed for high pressure oxygen compatibility. Forexample, a Rix Industries, Benicia, Calif., 1/3 hp unit costs about$10,000 while a Haskel International, Burbank, Calif., air-poweredbooster costs about $2200 in addition to requiring a compressed airsupply to drive it. Litton Industries and others also make oxygenpressure boosters.

Turning now to the liquid oxygen storage option, its main drawback isthat it requires a base reservoir--a stationary reservoir base unitabout the size of a standard beer keg--which has to be refilled aboutonce a week. The liquid oxygen can then be obtained from a base unit andtransferred to portable dewars which can be used by ambulatory patients.Also, with the liquid oxygen option, there is substantial waste, as acertain amount of oxygen is lost during the transfer to the portablecontainers and from evaporation. It is estimated that 20% of the entirecontents of the base cylinder will be lost in the course of two weeksbecause of losses in transfer and normal evaporation. These units willtypically boil dry over a period of 30 to 60 days even if no oxygen iswithdrawn.

There are other complications. Typically, supplemental oxygen issupplied to the patient by a home care provider, in exchange for whichit receives a fixed monetary payment from insurance companies orMedicare regardless of the modality. Oxygen concentrators for use in thehome are preferred and are the least expensive option for the home careprovider. For outside the home use however, only small high pressure gasbottles and small liquid dewars are portable enough to be used forambulatory needs. One of these two modalities may be used for bothin-home and ambulatory use or may be combined with an oxygenconcentrator which would provide in-home use. In either case, the homecare provider must make costly weekly or biweekly trips to the patient'shome to replenish the oxygen. One of the objects of this invention is toeliminate these costly "milk runs."

Portable oxygen concentrators are commercially available for providingpatients with gaseous oxygen. These devices are "portable" solely in thesense that they can be carried to another point of use such as in anautomobile or in an airplane. At present, there are no home oxygenconcentrators commercially available that can provide liquid oxygen. Onetype of medical oxygen concentrator takes in air and passes it through amolecular sieve bed, operating on a pressure swing adsorption cycle,which strips most of the nitrogen out, producing a stream of ˜90%oxygen, for example, as shown in U.S. Pat. Nos. 4,826,510 and 4,971,609(which are incorporated herein by reference). While, as set out in theInformation Disclosure Statement, complex oxygen liquefaction systemshave been disclosed for use by the military in jet aircraft, and whilelarge-scale commercial plants have been disclosed, this technology hasnot yet found its way into the home to help individual patients and tobenefit the general public. A truly portable oxygen concentrator has notyet been perfected and this event is unlikely, at least in the nearfuture, because the power requirements are too large to be provided by alightweight battery pack.

Since liquid oxygen requires periodic delivery and home oxygenconcentrators are not commercially available that would create liquidoxygen, there has existed a long-felt need for a device or method havingthe capability to concentrate oxygen from the air, liquefy it, andtransfer it into portable dewars in a home environment, and for a homeoxygen concentrator unit which allows excess flow capacity from theconcentrator to be stored by either compression or liquefaction forlater use.

SUMMARY OF THE INVENTION

The present invention presents a much safer and less expensive way ofproviding portable oxygen for patients who do not want to be tied to astationary machine or restricted by present oxygen technology. In onepreferred embodiment, the present invention splits off some of theexcess capacity gas flow from a PSA (pressure swing adsorption) ormembrane gas concentrator which has a relatively stable base load. Thissmall portion of the excess flow capacity, about one liter per minute(˜1 LPM) is stored via liquefaction. The stored gas can then be used asa portable supply away from the location of the gas concentrator. Thedaily six hour range capacity for a two liter per minute patient can beaccumulated by liquefying a one liter per minute gas flow for less than24 hours. Therefore, the entire daily requirement for mobility can beproduced every day if needed.

A summary of one of the many representative embodiments of the presentinvention is disclosed including a home liquid oxygen ambulatory systemfor supplying a portable supply of oxygen, where a portion of thegaseous oxygen output obtained from an oxygen concentrator is condensedinto liquid oxygen, comprising: (a) an oxygen concentrator whichseparates oxygen gas from the ambient air; (b) an outlet flow line totransfer flow of oxygen gas from said oxygen concentrator for patientuse; (c) a valve placed in the outlet flow line for splitting off aportion of the oxygen gas flow generated by the oxygen generator; (d) agenerally vertically oriented, gravity assisted, condenser for receivingand liquefying the split off portion of the oxygen gas flow; (e) acryocooler associated with said condenser; (f) a first storage dewar influid communication with said condenser for storing the oxygen liquefiedby the condenser, the first storage dewar having an outlet selectivelyengageable to and in fluid communication with at least one secondsmaller dewar and a fluid path for supplying liquid oxygen from thefirst dewar to the second dewar; (g) a heater for heating said firststorage dewar; (h) a controller for monitoring (i) oxygen concentrationof the oxygen gas flowing from said concentrator, and (j) the amount ofliquid oxygen in said first dewar, and for controlling the parameters ofliquid oxygen generation and transfer from said first storage dewar.

Another representative embodiment is the feature where the flow rateinto the condenser is chosen to exceed the capacity of the condenser. Inparticular, only 20 to 90% of the incoming flow into the condenser iscondensed to minimize the liquefaction of argon, nitrogen and tracegases, and to purge the system.

Additionally, the controller may control condenser parameters so thatthe condenser temperature varies in the range from approximately 69.2 to109.7 K, the condenser pressure varies from approximately 5 to 65 psia,and the concentrations of gas into the condenser varies with the oxygenrange being 80 to 100%, the nitrogen range being 0 to 20%, and the argonrange being 0 to 7%.

A unique condenser design is also disclosed where the condenser is inthermal contact with a cryocooler for use in liquefying oxygen andcomprises: (a) an inlet conduit for receiving oxygen; (b) an outermember; (c) an inner member; (d) a passage defined by said outer andinner members; (e) said inner member having radial slots to passages;(f) means for circulating said oxygen in said condenser.

Also disclosed is a representative method for controlling a homeambulatory liquid oxygen system comprised of an oxygen concentrator, acontroller having a microprocessor, a condenser, a cryocooler and astorage dewar, where all or only a portion of the oxygen flow isutilized for liquefaction, comprising: (a) providing the microprocessorwith a database and control functions; (b) sensing the parametersrelating to the concentration and supply of gaseous oxygen, the level ofliquid oxygen in the dewar, and the pressure of the condenser; (c)providing the microprocessor with these sensed parameters and having themicroprocessor calculate optimal conditions; (d) controllingservomechanisms to regulate the system so that optimal conditions arerealized as a function of said calculations.

The feature is also described wherein the liquid dewar will beperiodically boiled dry to eliminate any small amounts of water andhydrocarbons that may pass through the gas concentrator.

The above-summarized apparatus and methods, more specifically set out inthe claims, fill long-felt needs without posing any new safety issues tothe patient in that, for example, there are no potentially dangerouscanisters of high pressure compressed oxygen. The end result is thatpatients can ultimately use equipment with which they are familiar. Forexample, patients on liquid oxygen currently perform liquid transfersfrom large (30-50 liquid liter) dewars to small (0.5-1.2 liquidliter--corresponding to six hours of support) portable dewars. Thepresent invention will provide a means of supplying ambulatory oxygenfor a lower life cycle cost than the conventional method. Unlikeindustrial or military use liquefiers, which can take up whole rooms,the claimed oxygen liquefier (not including the oxygen concentrator)weighs less than 60 pounds and takes up less than six cubic feet ofvolume. There are currently about 700,000 patients in the United Statesusing ambulatory oxygen with an average yearly cost of about $1,960 perpatient. The estimated annual cost of oxygen per patient with thepresent invention is about $540.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the invention in the medical oxygenpreferred embodiment where gaseous oxygen is split off and liquefied forstorage in a stationary dewar or container.

FIG. 2 shows another preferred embodiment of the invention forambulatory supplemental oxygen using a portable LOX dewar.

FIG. 3 shows a typical temperature-composition diagram for oxygen-argonmixtures at typical dewar pressures.

FIG. 4 shows a typical temperature-composition diagram fornitrogen-oxygen mixtures at typical dewar pressures.

FIG. 5 shows typical oxygen concentration test data for gas streams intothe condenser and out of the dewar during liquefaction, and out of thedewar as the liquid is re-vaporized. (Oxygen source is a PSA oxygenconcentrator.)

FIG. 6 shows the controller block diagram for operation of the system.

FIGS. 7A through 7D are flow charts showing the controller logic.

FIG. 8 shows the controller input levels and output states for thestart-up mode of the preferred embodiment.

FIG. 9 shows the controller input levels and output states for thecondense mode of the preferred embodiment.

FIG. 10 shows the controller input levels and output states for thetransfer mode of the preferred embodiment.

FIG. 11 shows the controller input levels and output state for the boildry mode of the preferred embodiment.

FIG. 12 illustrates basic components of a pulse tube refrigerator.

FIG. 13 illustrates another embodiment of a cryocooler which may be usedin the subject invention.

FIG. 14 is a side cross-sectional view of the preferred embodiment ofthe condenser.

FIG. 15 is an end cross-sectional view of the preferred embodiment ofthe condenser corresponding to FIG. 14.

FIG. 16 shows a side cross-sectional view of another preferredembodiment of the condenser.

FIG. 17 shows an end cross-sectional view of another preferredembodiment of the condenser corresponding to FIG. 16.

FIG. 18 is an isometric view of the embodiment of the condenser shown inFIGS. 16 and 17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A flow chart of the preferred embodiment of the invention is set out inFIG. 1. Its main components include an oxygen concentrator 11, acryocooler 12, a condenser 13, and a storage/collection dewar or vacuuminsulated container 14. In the preferred embodiment, the oxygenconcentrator 11 operates on a pressure swing adsorption cycle andessentially strips all or most of the nitrogen from air along with otherminor components such as H₂ O, CO₂, CO, NO_(x), etc. The result is astream of dry gas with high oxygen concentration (˜92%) flowing in fluidoutlet 50. A portion of the gas from this output in fluid outlet 50 isrouted to a condenser 13 in association with a cryocooler (or cryogenicrefrigerator) 12 through flow lines 51 and 57. The cryocooler providescooling of the condenser heat exchanger 13 to liquefaction temperatures,causing the oxygen in contact therewith to go from the gaseous to theliquid phase. The condenser 13 typically must be insulated from ambientheating and may in practice even be located inside the dewar 14. Inorder to lessen the load on the cryocooler 12, a recuperator 15 may beused to pre-cool the incoming stream utilizing the vent flow throughline 52 out of the dewar as a cooling medium. In practice, thisrecuperator 15 may also be located within the dewar 14 to reduce ambientheating.

Controller 16 may be equipped with a microprocessor, adequate memory,software and ancillary equipment comprising a computer which can be usedto monitor and control the operation of the system. The controller 16may be provided with signals from liquid level sensor 17, oxygen sensor18, pressure transducer 9, and temperature sensor 10 via lines 53, 59,55 and 56, respectively. These signals are sensed and processed by thecomputer, with the controller operating valve 19, valve 25, heater 21,and cryocooler 12, in accordance with predetermined programs.

The controller also provides output indicators for the patient. Theliquid level in the dewar is continuously displayed and the patient isalerted when the oxygen concentration is low and when the system isready for them to transfer liquid to a portable dewar. A modem orwireless link may be included to enable remote monitoring of the keyparameters of the system by the home care provider as well asinformation which is useful for repair, maintenance, billing, andstatistical studies of patients for the medical oxygenation market. Keysystem parameters of interest include the number of liquid transfersperformed, the oxygen concentration history, number of run hours on thecryocooler, and time of the last boil-dry as well as number of boildries performed. The controller may include a computer and/or amicroprocessor located either integrally with the liquefaction systemclaimed herein or remotely therefrom but in communication therewithusing either a modem and telephone lines or with a wireless interface.The computer and/or microprocessor may include memory having a database,or may be remotely connected to a memory or database using a network. AnOptimal Liquefaction Schedule for optimal operation of the liquefactionsystem is set out in FIGS. 7-10 and may be stored in said controllerusing the memory and database. The controller can sense optimumparameters of the system and optimally control, including by activatingservomechanisms, liquefaction and transfer of liquid oxygen.

Dewar 14 is equipped with a dip tube 20 and heater 21. Heater 21 is usedto build pressure in the dewar in order to expel liquid out the dip tube20 when so desired. A quick disconnect valve 22 or other flow controlmeans is located on the end of the dip tube. This allows connection of aportable LOX dewar 23, which can then be carried by the patientrequiring a mobile/ambulatory supply of oxygen.

In another embodiment of this system shown in FIG. 2, the dewar 14 couldbe eliminated and replaced with a portable dewar 23 which is modifiedslightly from those existing today. The new portable dewar wouldinterface with the condenser 13, recuperator 15, and controller 16. Thisembodiment requires a slightly different control scheme from that givenfor the preferred embodiment as the transfer and boil-dry modes areeliminated. Any small amount of accumulated water and hydrocarbons areeliminated from the portable dewar 23 after each use by allowing it towarm to room temperature before reuse.

In operation, in the preferred embodiment of FIG. 1, where a pressureswing adsorption ("PSA") system is used, air is drawn into the oxygenconcentrator 11, where it is compressed by an oilless compressor,cooled, and passed through molecular sieve beds operating on thepressure swing adsorption cycle, as shown in U.S. Pat. Nos. 5,366,541;5,112,367; 5,268,021 and Re. 35,009, which are incorporated herein byreference. This PSA system produces a 5-6 liters per minute (LPM) gasstream with high oxygen concentration at 3-7 pounds per square inchgauge (psig). The composition of this gas stream varies but is typically90-95% oxygen, 5-6% argon, 0-4% nitrogen, <15 parts per million (ppm)water vapor, and <1 ppm hydrocarbons. Exhaust from the PSA cycle (80-84%nitrogen, 15-19% oxygen, 0.6-0.8% argon, and trace amounts of watervapor, carbon dioxide and hydrocarbons) is vented into the atmosphere aswaste gas. In the preferred embodiment, the high concentration oxygenstream in fluid outlet 50 is split with 0-4 lpm going through controlvalve 24, for patient consumption, and 0.5-1.5 lpm through line 51 andcontrol valve 19 for liquefaction. Oxygen sensor 18, monitors the oxygenconcentration produced by oxygen concentrator 11. If the oxygenconcentration falls below 88%, controller 16 will close valve 19 andturn off the cryocooler 12.

Even though 88% oxygen is adequate as supplemental oxygen therapy, ifthis was liquefied, as will be described below, the initial revaporizedstream may have a reduced oxygen content because of the close boilingpoints of the components of the mixture. The temperature of the splitgas stream entering the recuperator 15 is about room temperature. It iscooled to about 270 K (or colder) by the vent gas from the dewar flowingthrough the other side of the recuperator via line 52. The recuperator15 reduces the load on the cryocooler by using the cold vent gas topre-cool the oxygen-rich gas stream flowing into the condenser 13. Fromthe recuperator 15 the high oxygen concentration stream flows through aline 57 to the condenser 13, which is cooled to ˜90 K by the cryocooler12.

The condenser 13 provides cold surfaces to further cool and condense theflow. It is important to note that the gas passing through the condenser13 is a mixture of oxygen, argon, and nitrogen. The normal boilingpoints of these components are: 90.18 K, 87.28 K, and 77.36 Krespectively. Because of the close boiling points of the components ofthis mixture, there was initial skepticism because of the concern thatall the nitrogen and argon would condense along with the oxygen. If thisconcern was realized, when this liquid mixture was revaporized, thelower boiling point components; i.e., nitrogen and argon, would boil offfirst, resulting in flow with high concentrations of nitrogen, argon anda much lower oxygen concentration than that which was supplied to thecondenser--which would make the process of oxygen treatment ineffectiveor a failure.

This concern is explained in FIGS. 3 and 4 which are temperaturecomposition diagrams for binary mixtures of oxygen-argon andoxygen-nitrogen. In these diagrams taken from K. D. Timmerhaus and T. M.Flynn, Cryogenic Process Engineering, Plenum Press, 1989, pp. 296-297,the upper curve at a given pressure defines the dew point and the lowercurve defines the bubble point. Looking at FIG. 4 for a pressure of 0.101 MPa, if there is a gas mixture with 10 mole percent nitrogen (point1), condensation will start when the gas has cooled to the dew pointcurve (point 2g) which is at a temperature of about 89.5 K in this case.Because oxygen has a higher boiling point than nitrogen, the initialliquid formed (point 2f) will have only 7.4 mole percent nitrogen. Ifthe temperature is lowered to point 3, the liquid will have thecomposition of point 3f while the remaining vapor will have thecomposition of point 3g. As the temperature is lowered further to point4f or below, all of the mixture liquefies and the composition is 10 molepercent nitrogen, the same as at point 1. If this liquid is heated, thenitrogen which has a lower boiling point will vaporize first. Thus, thecomposition of the first vapor formed will be that of point 4g or about30 mole percent. As the remaining liquid boils, the mole percent ofnitrogen in the vapor drops back to 10 mole percent when point 2g isreached. It is believed that the composition swings with a ternarymixture of oxygen, argon and nitrogen will be even more pronounced thanthose shown in FIGS. 3 and 4 for binary mixtures. Fortunately, thisconcern was avoided when the system was set so that only 20 to 90% ofthe incoming flow to the condenser was actually condensed and when thecondenser was controlled in accordance with the parameters as explainedherein. This is believed to work because the excess flow operates topurge the vapor with higher impurity concentration from the system andavoid the aforementioned problem. Instead, the results realized werethat a high concentration stream of oxygen could be liquefied and storedas set out in the portable ambulatory device claimed herein.

FIG. 5 shows typical oxygen concentration test data for condensing andre-vaporizing part of the product outlet stream from the oxygenconcentrator 11 in the preferred embodiment. For the first 120 minutesafter the system was turned on, the system was cooling down without anynet liquid accumulation in the dewar 14. From this point up to about 500minutes, condensation continued with liquid accumulation. During thistime phase, the inlet stream to the condenser 13 had an oxygenconcentration of 95%, while the vent flow through line 52 had an oxygenconcentration of only 92-93%. After 500 minutes the inlet stream andcondenser cooling were stopped. The oxygen concentration of there-vaporized liquid increased as the liquid boiled off due to the lowerboiling point components (argon and nitrogen) boiling off first. Thischange in oxygen concentration presents no problem for medicalambulatory use because the oxygen concentration remains above 85%.

Because of the aforementioned mixture problem, it is important and evencritical not to let the amount of argon and nitrogen in the liquidbecome too high or when it is revaporized, the oxygen concentration willinitially be much lower than that conventionally used in supplementaloxygen therapy (>85%). This can be accomplished by selecting the propercondenser temperature, which is a function of pressure, and by notcondensing all of the incoming flow. If only part of the incoming flow(20-90%) is liquefied, the remainder of the flow will purge the vaporwith higher impurity concentration from the system. A condensertemperature of about 90 K (for ˜17 psia) minimizes the amount of argonand nitrogen liquefied without overly diminishing the yield of oxygen.Hence there will be both liquid and vapor leaving the condenser. Theliquid will fall into the dewar 14 and collect. The vapor which has notcondensed is vented to the atmosphere through line 52 and therecuperator 15.

The amount of incoming flow liquefied is controlled by setting the massflow rate relative to the cooling capacity of the cryocooler. Theparameters of the condenser and/or cryocooler can be stored in thememory of the controller and/or computer and the controller regulatingthe incoming flow depending on the parameters stored and/or sensed.Having a mass flow rate which exceeds the cooling capacity of thecryocooler/condenser combination, prevents the incoming flow from beingcompletely liquefied. The mass flow rate is controlled by the amount offlow restriction between inlet valve 19 and flow control valve 25. Thisincludes the flow losses of the valves themselves as well as those inthe recuperator, condenser, and all of the interconnecting plumbing.

The pressure in the dewar 14 is maintained slightly above ambientpressure while the cryocooler is operating by valve 25. It is desirableto keep the pressure in the condenser as high as possible because thisincreases the condensation temperature (as shown in FIGS. 3 and 4) whicheases the requirements on the cryocooler. Once again this can becontrolled by the controller and/or the computer, microprocessor andmemory system.

This pressure regulating function of the solenoid on-off valve 25 isaccomplished by the pressure transducer 9 and controller 16.Alternately, a back pressure regulating valve (such as a Tescom BB-3series) or a suitable servomechanism may be used in lieu of the activelycontrolled solenoid. Liquid keeps accumulating in the dewar 14 until theliquid level sensor 17 signals the controller that the dewar is full oruntil the oxygen sensor 18 signals that the oxygen concentration offluid exiting the oxygen concentrator 11 is too low.

In the best mode, operating parameters for optimal operation of thesystem for the condenser should be that the condenser surfacetemperature should be in the range from 69.2-109.7 K and pressure shouldbe in the range from 5-65 psia. The gas concentrations into thecondenser for medical use should have oxygen in the range of 80-100%,nitrogen from 0-20%, and argon from 0-7%.

In order to transfer liquid from the dewar 14; e.g. to fill a portableLOX dewar 23, the pressure in the dewar 14 must be increased so thatliquid can be forced up the dip tube 20. As shown in FIG. 1, heater 21is used for this purpose. Heater 21 may be immersed in the liquid oxygenor attached to the outer surface of the inner vessel. The controller 16ensures that the cryocooler 12 is turned off and valve 25 is closedbefore the heater 21 is energized. The heater 21 remains turned on untilthe pressure, measured by pressure transducer 9, reaches about 22 psig.

In order to eliminate accumulation of solid water and hydrocarbons whichmay be supplied in trace amounts from the oxygen concentrator, the dewar14 will be warmed to room temperature periodically (preferably afterabout 30 fillings of a portable dewar, or every two months). Thisprocedure is accomplished most economically when the inventory of liquidin the storage dewar is low; e.g. shortly after liquid transfer and aportable dewar has been filled. In this "boil-dry" mode, valve 19 willbe closed, the cryocooler 12 is turned-off, valve 25 is open, and heater21 is energized. The heater will boil-off the remaining liquid in thedewar and with it any trace amounts of water and hydrocarbons which arecondensed and solidified in the liquid oxygen or on the cold surfaces.The heater 21 will remain turned on until the dewar temperature,measured by temperature sensor 10, has warmed to about 300 K. Anyremaining water vapor will be flushed out by gaseous oxygen during thesubsequent cool-down.

At initial start-up or after a periodic boil-dry phase, the dewar,condenser, recuperator, and all associated hardware are at roomtemperature and must be cooled down. This is accomplished in the"start-up" mode, where valve 19 (see FIG. 1) is open, the heater is off,the cryocooler is on, and valve 25 is modulated to control thepressure/flow rate. It is desired to keep the pressure and hence thedensity of the gas as high as possible while maintaining the flow rate.

The higher density gas will have better heat transfer with the dewarwalls and associated hardware. It is noted that higher flow rates willenhance the convection heat transfer but may exceed the coolingcapacity. Based on the cooling characteristics of the cryocooler betweenroom temperature and 90 K, the flow rate can be changed to minimize thecool-down time.

The dewar 14 is equipped with at least one relief valve 26 as a safetyfeature. Another relief valve 29 is provided and in communication withthe inlet gas stream 51, before flowing into the recuperator 15. Thisserves as a back-up for relief valve 26 as well as providing a means toeliminate accumulated water from the recuperator 15 during periods whenthe cryocooler 12 is off, if valve 25 is closed. A check valve 27 isalso provided to prevent backflow into the oxygen concentrator in theevent of a malfunction.

FIG. 6 provides a block diagram of the controller 16 control system withsensor input value ranges and output states. It also shows interfaces toan indicator and a modem or wireless interface. The mode switch 28 maybe used by the patient to request the system to prepare for a liquidtransfer to a portable dewar. The indicator then provides a visualsignal that the system is building pressure in the dewar. Once thepressure has reached the desired value, a visual and/or audio signal isgiven to alert the patient that the system is ready to transfer liquid.The controller may also be programmed to perform an unattended liquidtransfer. The modem, telephone line or wireless interface connectionsare optional hardware that may be added to the controller to enableremote monitoring of the system by the home care provider (e.g., toassist with maintenance and repair) or insurance companies or healthproviders/administrators (e.g., to assess if patients are using enoughambulatory oxygen to justify payments, etc.).

FIGS. 7A-D show a logic flow chart for the controller for the normaloperation modes. This can also be referred to as the "input/outputcontrol schedule." The mode switch 28 can also be used by a repair orfactory technician to put the controller in a calibration mode whichserves as a method to check and reset the program. As shown in FIG. 6,the indicator provides liquid level readout, transfer request status,and low oxygen concentration information to the patient. All of thesensors are continuously scanned to provide the controller with thelatest information. FIGS. 8 through 11 provide detailed output states asa function of input levels for the normal operating modes (start-up,condense, transfer, and boil-dry), which can be referred to as the"Optimal Liquefaction Operational Schedule."

For example, FIG. 8 relates to the start-up mode; i.e., when the systemis first turned on or after the boil-dry cycle. As shown in FIGS. 6through 8 and as depicted in FIG. 1, at start-up mode, the liquid sensor17 shows zero liquid volume in the dewar and, when the oxygen sensor 18shows an oxygen concentration greater than 88%, valve 19 is open, heater21 is off, cryocooler 12 is on, and the indicator or the controllerindicates a cool-down state. Valve 25 is modulated to control pressure.

Once the system attains a cool enough temperature, steady state ornormal operational condense mode is used. As shown in FIG. 9, the inputto the controller 16 is such that when the oxygen sensor indicatesoxygen concentration being greater than 88% and when the other criteriain the left-hand column of FIG. 9 are achieved, the output states setout in the right portion of the chart are attained. For example, whenthe level of the dewar is sensed as being full, the liquid level sensorindicates a level of approximately 100%, causing closure of valves 19and 25, keeping the heater off, turning the cryocooler off, and havingthe indicator signal that the dewar is full.

The transfer mode in FIG. 10 is the stage where one can fill theportable thermos bottles or dewars 23 from the main storage dewar 14.The top portion of FIG. 10, for example, shows controller readoutswhere, if the liquid sensor indicates a liquid level of less than 20%,then the conclusion is computed that there is not enough liquid totransfer into the portable dewar from the main dewar as shown. When theoperator wants to increase the pressure in the storage dewar to forcethe liquid oxygen into the portable dewar, the heater is activated, andwhen the pressure sensor indicates that the pressure exceeds 22 psig, asshown on the last line in the left-hand column of FIG. 10, the heater 21is then turned off and the controller readout or indicator shows thatthe transfer of liquid oxygen can be made to the portable dewar.Finally, FIG. 11 indicates the boil-dry mode, with valve 25 open toallow the vapor to escape, and the various parameters relating thereto.

FIG. 12 shows a schematic of a pulse tube refrigerator, the preferredembodiment of the cryocooler 12 in FIG. 1. Because the cooling load onthe condenser is small (7-15 W), the pulse tube refrigerator ispreferred for use in the subject ambulatory oxygen system because of itsgood efficiency with only one moving part, the pressure oscillator.Pulse tube refrigerators are shown in U.S. Pat. Nos. 5,488,830;5,412,952 and 5,295,355 the disclosure of which are hereby incorporatedby reference. FIG. 11 depicts a pulse tube refrigerator of the doubleinlet type. Other types of pulse tube refrigerators (PTR) could also beused such as the basic PTR or the inertance tube PTR (Zhu et al., 9^(th)International Cryocooler Conference, NH, June 1996).

The double inlet pulse tube refrigerator as shown in FIG. 12 iscomprised of a pressure oscillator 30, primary heat rejector 31,regenerator 32, heat acceptor 33, pulse tube 34, orifice rejector 35,bypass orifice 36, primary orifice 37, and reservoir volume 38. Thepreferred refrigerant gas in the PTR closed and pressurized circuit ishelium but various other gases such as neon or hydrogen could also beused. In operation, the PTR essentially pumps heat accepted at lowtemperature in the heat acceptor 33 to the orifice heat rejector 35where it is rejected at near ambient temperature. Although FIG. 12depicts a "U-tube" configuration of the PTR, in-line and coaxialconfigurations are other possible options. Depicted therein is a pistontype pressure oscillator, but other types are possible such as thoseutilizing diaphragms or bellows.

FIG. 13 shows a schematic of another embodiment of the cryocooler. Thisis a vapor compression cycle cryocooler using a mixed gas refrigerantsuch as shown in U.S. Pat. No. 5,579,654; a 1969 German Patent byFuderer & Andrija; British Patent No. 1,336,892. Other types ofcryocoolers will work as long as they meet the important criteria ofsmall size, convenience and low cost. In FIG. 13, the refrigerant iscompressed by the compressor to high pressure. Then it is cooled by theaftercooler with heat Qh being rejected to the environment. Oil isseparated in the oil separator. Oil flows back to the compressor inletthrough a flow restriction. The refrigerant gas flows to a heatexchanger where it is cooled by the returning cold stream. Somecomponents of the mixture may condense in this process. The liquid/gasrefrigerant mixture flows through a throttle valve where its pressure isreduced and its temperature drops. This cold refrigerant enters theevaporator where the heat load Qc is absorbed and some liquid is boiledinto vapor. This vapor flows up the cold side of the heat exchangerabsorbing heat from the incoming stream. Then it flows back to thecompressor. Heat Qc is accepted at cold temperature Tc. This is wherethe condenser would interface with the cryocooler.

It is noted that with this type of cryocooler, it may be possible toremove some of the heat from the oxygen stream at a temperature warmerthan Tc.

One possible geometry of the generally vertically oriented, gravityassisted condenser 13 in FIG. 1 is shown in FIGS. 14 and 15. Theincoming gas from the oxygen concentrator flows from conduit 57 tochamber 58 and then is distributed through an annular passage 59 betweenthe outer tube 41 and inner rod 42. The inner rod 42 is made of a highthermal conductivity material such as OFHC (Oxygen Free HighConductivity) copper, to minimize the temperature gradient between thesurface on which the oxygen condenses (13) and the cryocooler 12. Thecold end of the cryocooler is shown by cross-hatched member 61. Due tosurface tension, the axial slots or grooves 43 will draw in liquid as itcondenses. This will enhance heat transfer from the incoming gas bypreventing a liquid film from forming over the entire condenser surface.Condensed liquid will drip off the bottom of the rod 42 whilenon-condensed gases flow out the end of the annulus 60. It is possibleto liquefy all of the incoming flow to the condenser provided thecryocooler has sufficient cooling capacity and temperature capability.However, in order to minimize the amount of nitrogen and argoncondensed, the preferred embodiment only condenses between 20-90% of theincoming flow. The incoming flow rate can be determined by theappropriate sizing of flow restrictions downstream of and by controllingvalve 19. As mentioned previously, the mass flow rate is chosen toexceed the cooling capacity of the condenser/cryocooler so that onlypart of the incoming flow is liquefied. Also, the pressure in thecondenser is maintained as high as possible while maintaining thedesired flow rate. The higher pressure increases the condensationtemperature which in turn reduces the requirements on the cryocooler.

FIGS. 16 and 17 show another embodiment of the condenser that allowseasier integration with the mixed gas refrigerant cryocooler. Thisconfiguration also allows access to the liquid in the dewar through thecenter of the condenser. The cold end of the cryocooler 45 is in thermalcontact with an outer tube 46 and an inner tube 47, both of which aremade of a high thermal conductivity material such as OFHC copper andwhich utilize flanges 62 and 64 to interface with the cryocooler. Theinner tube has axial slots or grooves 48 cut into its outer surface(see, FIG. 17) to increase the surface area and to wick condensedliquid, preventing a liquid film from forming over its entire surface.Gas enters the condenser through port 63. The liquid and vapor flow downthrough an annular passage 49. An isometric view of this embodiment isshown in FIG. 18.

Thus, an improved home/ambulatory liquid oxygen system is disclosed.While the embodiments and applications of this invention have been shownand described, and while the best mode contemplated at the present timeby the inventors has been described, it should be apparent to thoseskilled in the art that many more modifications are possible, includingwith regard to scaled-up industrial applications, without departing fromthe inventive concepts therein. Both product and process claims havebeen included and in the process claims it is understood that thesequence of some of the claims can vary and still be within the scope ofthis invention. The invention therefore can be expanded, and is not tobe restricted except as defined in the appended claims and reasonableequivalence departing therefrom.

We claim:
 1. A home liquid oxygen ambulatory system for supplying aportable supply of oxygen, where a portion of gaseous oxygen outputobtained from an oxygen concentrator is condensed into liquid oxygen,comprising:(a) an oxygen concentrator which separates oxygen gas fromthe ambient air; (b) an outlet flow line to transfer flow of oxygen gasfrom said oxygen concentrator for patient use; (c) a valve placed in theoutlet flow line for splitting off a portion of the oxygen gas flowgenerated by the oxygen generator; (d) a condenser for receiving andliquefying the split off portion of the oxygen gas flow; (e) acryocooler associated with said condenser; (f) a first storage dewar influid communication with said condenser for storing the oxygen liquefiedby the condenser, the first storage dewar having an outlet selectivelyengageable to and in fluid communication with at least one secondsmaller dewar and a fluid path for supplying liquid oxygen from thefirst dewar to the second dewar; (g) a heater for heating said firststorage dewar; (h) a controller for monitoring the amount of liquidoxygen in said first dewar, and for controlling the parameters of liquidoxygen generation and transfer from said first storage dewar.
 2. Thehome liquid oxygen ambulatory system of claim 1 wherein the oxygenconcentrator is a pressure swing adsorption ("PSA") type oxygenconcentrator.
 3. The home liquid oxygen ambulatory system of claim 1wherein the flow rate into the condenser is chosen to exceed thecapacity of the condenser.
 4. The home liquid oxygen ambulatory systemof claim 1 wherein only 20 to 90% of the split off portion flowing intothe condenser is condensed to minimize the liquefaction of argon,nitrogen and trace gases.
 5. The home liquid oxygen ambulatory system ofclaim 1 wherein the controller controls condenser parameters so that thecondenser temperature varies in the range from approximately 69.2 to109.7 K, the condenser pressure varies from approximately 5 to 65 psia,and the concentrations of gas into the condenser varies approximately asfollows:oxygen: 80 to 100% nitrogen: 0 to 20% argon: 0 to 7%.
 6. Thehome liquid oxygen ambulatory system of claim 1 wherein vent gas fromthe first storage dewar is recovered and used to pre-cool the gas intothe condenser.
 7. The home liquid oxygen ambulatory system of claim 1wherein the condenser is in thermal contact with the cryocooler andcomprises:(a) an inlet conduit for receiving the split off portion ofoxygen; (b) an outer member; (c) an inner member having a chamber incommunication with said inlet conduit; (d) a passage defined by saidouter and inner members; (e) said inner member having axial slots; and(f) means for circulating said oxygen in said passage defined by saidinner and outer member.
 8. The home liquid oxygen ambulatory system ofclaim 1, further comprising a recuperator interfaced between the oxygenconcentrator and the condenser.
 9. The home liquid oxygen ambulatorysystem of claim 1, wherein the controller senses pressure in said firstdewar and controls said heater in response thereto.
 10. The home liquidoxygen ambulatory system of claim 1 wherein the first storage dewar willbe periodically boiled dry to eliminate any trace gases that may passthrough the gas concentrator.
 11. The home liquid oxygen ambulatorysystem of claim 1, wherein the condenser comprises a generallyvertically oriented, gravity assisted, circular housing with an innercenter grooved core defining an annulus and with axial slits to wickaway the condensed liquid.
 12. The home liquid oxygen ambulatory systemof claim 1 wherein the controller is located remotely from the oxygensource.
 13. The home liquid oxygen ambulatory system of claim 1 whereinthe controller monitors the oxygen concentration and amount of liquidoxygen and controls the parameters of liquid oxygen generation andtransfer remotely using a modem.
 14. The home liquid oxygen ambulatorysystem of claim 1 wherein the controller monitors the oxygenconcentration and amount of liquid oxygen and controls the parameters ofliquid oxygen generation and transfer remotely using a wirelessinterface.
 15. The home liquid oxygen ambulatory system of claim 1,wherein the controller monitors the oxygen concentration of the oxygengas flowing from said concentrator.
 16. The home liquid oxygenambulatory system of claim 1, further including an oxygen sensor tomeasure the oxygen concentration of flow from said concentrator, aliquid level sensor in said first dewar, a temperature sensor in saidfirst dewar, a pressure sensor in communication with said first dewar,and wherein the controller receives output from the oxygen sensor,liquid level sensor, temperature sensor and pressure sensor, andcontrols the flow of input gaseous oxygen to the condenser and controlsthe liquefaction and transfer from the first storage dewar in accordancewith the sensed conditions.
 17. A home liquid oxygen ambulatory systemfor supplying a portable supply of oxygen, where a portion of gaseousoxygen output obtained from an oxygen concentrator is condensed intoliquid oxygen, comprising:(a) an oxygen concentrator which can isolateoxygen gas from the air; (b) output means from the concentrator forchanneling the oxygen which is isolated; (c) means for splitting off allor a portion of the gaseous oxygen from the output means of saidconcentrator for liquefaction; (d) a condenser which can cause gaseousoxygen to change phase into liquid oxygen; (e) means for channeling theoxygen stream from said output means to said condenser; (f) means forliquifying said oxygen stream in said condenser using a cryocooler; and(g) means for collecting the condensed liquid oxygen using a firstdewar, wherein said first dewar includes a heater which can be used toeffectuate the transfer of liquid oxygen from the first dewar to asecond dewar for storing a quantity of liquid oxygen from which smallerquantities can be transferred for moveable oxygen treatment.
 18. Thehome liquid oxygen ambulatory system of claim 17 wherein the oxygenconcentrator is a pressure swing adsorption ("PSA") type oxygenconcentrator.
 19. The home liquid oxygen ambulatory system of claim 17wherein the flow rate into the condenser is chosen to exceed thecapacity of the condenser.
 20. The home liquid oxygen ambulatory systemof claim 17 wherein only 20 to 90% of the incoming flow to the condenseris condensed to minimize the liquefaction argon, nitrogen and tracegases.
 21. The home liquid oxygen ambulatory system of claim 17 whereinthe controller controls condenser parameters or ranges so that thecondenser temperature varies in the range from approximately 69.2 to109.7 K and the condenser pressure varies from approximately 5 to 65psia, and the concentrations of gas into the condenser variesapproximately as follows:Oxygen: 80 to 100% Nitrogen: 0 to 20% Argon: 0to 7%.
 22. The home liquid oxygen ambulatory system of claim 17 whereinvent gas from the storage dewar is recovered and used to pre-cool thegas into the condenser.
 23. The home liquid oxygen ambulatory system ofclaim 17, further comprising a recuperator interfaced between the oxygenconcentrator and the condenser.
 24. The home liquid oxygen ambulatorysystem of claim 17, further comprising a controller which sensespressure in the first dewar and controls said heater in responsethereto.
 25. The home liquid oxygen ambulatory system in claim 24wherein the controller is located remotely from the oxygen generator.26. The home liquid oxygen ambulatory system in claim 24 wherein thecontroller monitors the oxygen concentration and amount of liquid oxygenand controls the parameters of liquid oxygen generation and transferremotely using a modem.
 27. The home liquid oxygen ambulatory system inclaim 24 wherein the controller monitors the oxygen concentration andamount of liquid oxygen and controls the parameters of liquid oxygengeneration and transfer remotely using a wireless interface.
 28. Thehome liquid oxygen ambulatory system of claim 17 wherein the liquiddewar will be periodically boiled dry to eliminate any trace gases thatmay pass through the gas concentrator.
 29. The apparatus of claim 17,wherein the generally vertically oriented, gravity assisted condensercomprises a circular housing with an inner center grooved core definingan annulus and with axial slits to wick away the condensed liquid. 30.The home liquid oxygen ambulatory system of claim 17, further includingan oxygen sensor to measure the oxygen concentration of flow from saidconcentrator, a liquid level sensor in said dewar, a temperature sensorin said dewar, a pressure sensor in communication with said dewar, and acontroller that receives output from the oxygen sensor, liquid levelsensor, temperature sensor and pressure sensor, and controls the flow ofinput gaseous oxygen to the condenser and controls the liquefaction andtransfer from the storage dewar in accordance with the sensedconditions.